Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for monitoring a tension that appears in an antenna that is towed in water by a vessel.
Discussion of the Background
Reflection seismology is a method of geophysical exploration to determine the properties of a portion of the earth's subsurface, information that is especially helpful for determining the location of underground deposits (e.g., oil, gas, etc.). Marine reflection seismology is based on the use of a controlled source that sends energy waves into the earth. By measuring the time it takes for the reflections to come back to plural receivers, it is possible to estimate the depth and/or composition of the features causing such reflections. These features may be associated with the underground deposits.
For marine applications, a conventional seismic survey system 100, as illustrated in FIG. 1, includes a vessel 102 that tows plural streamers 110 (only one is shown in the figure) and a seismic source 130. Streamer 110 is attached through a lead-in cable (or other cables) 112 to vessel 102, while source array 130 is attached through an umbilical 132 to the vessel. A head float 114, which floats at the water surface 104, is connected through a cable 116 to a head end 110A of streamer 110, while a tail buoy 118 is connected, through a similar cable 116, to a tail end 110B of streamer 110. Head float 114 and tail buoy 118 are used, among other things, to maintain the streamer's depth. Seismic receivers 122 are distributed along the streamer and are configured to record seismic data. Seismic receivers 122 may include a hydrophone, geophone, accelerometer, gradient pressure receiver or a combination thereof. Positioning devices (birds) 128 (only one shown for simplicity) are attached along the streamer and controlled by a controller 126 for adjusting a position of the streamer according to a survey plan.
Source array 130 has plural source elements 136, which typically are air guns. The source elements are attached to a float 137 to travel at desired depths below the water surface 104. During operation, vessel 102 follows a predetermined path T while source elements 136 emit seismic waves 140. These waves bounce off the ocean bottom 142 and other layer interfaces below the ocean bottom 142 and propagate as reflected/refracted waves 144 that are recorded by receivers 122. The positions of both the source element 136 and recording receiver 122 are estimated based on GPS systems 124 and recorded together with the seismic data in a storage device 127 onboard the vessel. Controller 126 has access to the seismic data and may be used to achieve quality control or even full processing of this data. Controller 126 may be also connected to the vessel's navigation system and other elements of the seismic survey system, e.g., birds 128.
During the seismic acquisition process, the tensile loads of the towed seismic streamers have to be monitored, especially during deployment and retrieval operations. This is so because a higher tensile load on the streamer may rupture or damage or break the streamer. Currently, the tensile load is measured by an in-line tensile module 150 (see FIG. 1) inserted between the lead-in 112 and the first active section 110C of the streamer. The in-line tensile module includes a tensile load sensor that can measure the tensile load generated by towing the entire streamer in water. However, this module allows measuring the tensile load only in one location of the streamer.
Another problem with the existing in-line tensile modules is that, during the deployment and retrieval phases of the streamer, the tensile module may be wrapped onto the winch on the vessel and thus, unable to provide any relevant tensile load measurements.
Thus, there is a need to provide tensile module load information about a towed cable in water, even when part of the towed cable is on the vessel.